The present disclosure relates to an antireflection film, a method for heating a metal film, and a heating apparatus.
For example, when light traveling in air is incident on a medium, reflection occurs at a light incident surface of the medium. This reflection is caused by the difference between the refractive index of air and that of the medium. As a method for suppressing the reflection as described above, a technique has been used in which a single-layer or a multilayer antireflection film is provided on the light incident surface of the medium, such as a surface of a glass substrate, by a vacuum deposition method or the like.
In particular, for example, in Japanese Patent No. 2590133, an antireflection film shown in the following Table 1 has been disclosed. In addition, for example, in Japanese Unexamined Patent Application Publication No. 1-168854, an antireflection film shown in the following Table 2 has been disclosed.
TABLE 1Film ThicknessRefractive Index(Å)(Part I)Low refractive-index dielectric film1.35~1.50  700~1,200High refractive-index dielectric film1.90~2.50200~400Metal film20~60Transparent substrate1.45~1.60(Part II)Low refractive-index dielectric film1.35~1.50  700~1,200High refractive-index dielectric film1.90~2.501,000~1,400Metal film10~40Transparent substrate1.45~1.60
TABLE 2RefractiveFilm ThicknessIndex(× λ0/4)Second low refractive-index dielectric film1.37~1.500.8~1.2Second high refractive-index dielectric film2.00~2.400.03~0.80Metal film35~90 ÅFirst low refractive-index dielectric film1.37~1.501.40~1.80First high refractive-index dielectric film2.00~2.400.30~0.50Transparent substrate1.40~1.70
Incidentally, for example, when a metal film made of molybdenum is directly deposited on a glass substrate (refractive index: 1.51), a reflectance R at an interface between the glass substrate and the metal film is as shown below. In this case, nM and kM of a complex refractive index represented by NM=nM−i·km of the metal film made of molybdenum are 3.53 and 3.3, respectively.R={(1.51−3.53)2+3.32}/(1.51+3.53)2+3.32=0.41
When the reflectance R at the interface is high as described above, energy of laser light, which is incident on the glass substrate, reaches the metal film, and is absorbed therein, is decreased to approximately half of original power of a laser light source, and in addition, strong reflected light returns to the laser light source. As a result, laser oscillation becomes unstable, and in the worst case, the laser light source may be damaged. In the antireflection film disclosed in the above Japanese Patent No. 2590133, since the antireflection film is not provided between the glass substrate and the metal film, light which is incident on the glass substrate and which reaches the metal film may not be prevented from being reflected by the metal film. In addition, in the antireflection film disclosed in Japanese Unexamined Patent Application Publication No. 1-168854, it is attempted to prevent light incident on the second low refractive-index dielectric film from being reflected by the transparent substrate, and whether reflection of light which is incident on the glass substrate and which reaches the metal film can be prevented has not been described at all.